The present invention relates to the dynamic channel allocation (DCA) for a cellular radiocommunication networks such as a GSM-type network.
A distinction is made between interference-adapted DCA schemes and traffic-adapted DCA schemes. The interference-adapted schemes take account of the quality of the radio signals received, and dynamically allocate the channels assumed to have the lowest noise. The traffic-adapted schemes are based on the knowledge of which channels are used in each cell in order to ensure that two identical channels are not allocated in adjacent cells: they do not rely on any radio quality criterion but require information to be exchanged between the different channel-allocating units within the network.
In the field of the interference-adapted DCA algorithms, the algorithms proposed so far are designed to allocate channels on the radio link for packet mode communications (see M. FRULLONE et al: xe2x80x9cDynamic Channel Allocation for ATDMAxe2x80x9d, Proc. of the Race Summit, Lisbon, Nov. 1995, pages 299-203). This is not suitable for networks which support communications in circuit mode, such as GSM. As for the traffic-adapted DCA algorithms, the studies that have been conducted are basically theoretical. Due to the fact that these mechanisms require large exchanges of signaling between the different entities allocating radio resources within the network, they are not particularly well suited to current cellular networks, which do not facilitate such exchanges. At present, therefore, they are of little interest.
These days, mobile network operators apply channel planning techniques as a means of distributing the physical channels to be allocated amongst the different cells of the network. The term xe2x80x9cplanningxe2x80x9d means that each cell is allocated its own list of physical channels from which channels are selected as they are allocated. In the majority of systems, the channel planning is simplified to a frequency planning.
The advantage of such a technique is that it is possible to ensure that no channel is allocated to two adjacent cells, which reduces the risk of interference between two channels allocated in adjacent cells. This ensures a certain radio quality on each channel allocated in the network.
However, frequency planning has the following disadvantages:
1) Its implementation is fastidious. The more irregular the topology of the cellular coverage is (different cell sizes, non-symmetrical patterns, . . . ), the more difficult it is to plan. The concept of a multicellular network with microcells and xe2x80x9cumbrellaxe2x80x9d macrocells makes any attempt at frequency planning even more difficult since it introduces several levels of coverage, each requiring a frequency plan. This also has the effect of restricting the number of frequencies allocated to each cell (thereby limiting traffic).
2) Frequency planning does not make it easy to change the network topology. For example, each time base stations are added to or withdrawn from the network, a new frequency plan is needed over a significant part of the network. This aspect is becoming even more complex now that operators are required to modify their cellular engineering on a frequent basis (integrating microcell zones into the existing network).
3) Frequency planning does not allow the system to allocate resources to mobiles in a flexible manner. In view of the fixed (and limited) number of resources allocated by cells, the system is unable to absorb local traffic peaks due to the lack of available resources.
Whilst frequency planning may remain an interesting solution in a macro-cellular environment (simple and regular cell configuration, homogeneous traffic distribution), it is less so when it comes to other types of environment (micro-cellular, xe2x80x9cumbrellaxe2x80x9d cells, traffic peaks, . . . ).
The sharp increase currently seen in mobile communications traffic due to the success of portable telephones has forced operators to increase the density of their networks. They have therefore now been forced to combine any type of cellular configuration (macrocells, microcells, umbrella cells, omnidirectional antenna, directional antenna, . . .) and accordingly need a more flexible mechanism for allocating channels than frequency planning.
WO96/31075 discloses a method of dynamically allocating channels for a cellular radio network in which xe2x80x9cstatistical preferencesxe2x80x9d, i.e. priorities, are assigned to different frequency channels within a same cell. The selection of which frequency channel is to be used is based on these xe2x80x9cstatistical preferencesxe2x80x9d. The process of determining xe2x80x9cstatistical preferencesxe2x80x9d is based on measurements of channel characteristics, taken when these channels are not being used.
The DCA mechanism disclosed in this document WO96/31075 does not use radio measurements taken during a communication. The same applies to the mechanism described in U.S. Pat. No. 5,507,008. According to the latter document, when a call is being established, the base station of a cell checks that the envisaged channel is not subject to too much interference. If the channel-to-interferer ratio (CIR) is too low, the base station moves on to the next channel in a general list common to all the cells.
GB-A-2,266,433 discloses another DCA mechanism in which several frequency lists are held for each cell. The base station determines a transmission loss of a signal returned by a mobile station in order to select a list of frequencies, from which the channel is selected on the basis of a quality criterion. This quality criterion may be based in particular on the channel-to-interferer ratio. The frequency lists are updated depending on the success encountered during previous attempts to select the channels concerned. Here again, no use is made of radio measurements taken during communication.
An object of the invention is to propose an efficient technique for allocating radio resources dedicated to communications, which does not require prior frequency planning and which therefore releases the operator from the above constraints.
The method is also intended to enable the network to absorb traffic peaks under certain conditions.
The invention thus proposes a method of dynamically allocating channels in a cellular radio network, wherein a set of physical channels is used in said network to form logical channels dedicated to communications in circuit mode between mobile stations and geographically distributed base stations, each logical channel belonging to a physical channel, and wherein, for each communication established on a logical channel between a base station and a mobile station, radio parameters representative of conditions of said communication on said logical channel are periodically measured. According to the invention, the method comprises the following steps, carried out for each base station:
associating a respective priority index to each physical channel of said set;
holding a first list of physical channels which are not being used by said base station to communicate with any mobile station, and at least a second list of busy physical channels, each having at least one active logical channel dedicated to a communication currently taking place between said base station and any mobile station;
updating the priority indices associated with the physical channels of the second list on the basis of the radio parameters measured in relation to communications taking place on the logical channels belonging to said physical channels of the second list; and
when establishing a communication with a mobile station, selecting for said communication an accessible and non-active logical channel belonging to a physical channel whose priority index is maximum.
As a result of this DCA mechanism, it is no longer necessary, before putting the network into service, to plan the allocation of frequencies between the cells in order to distribute between them the radio resources that can be allocated to mobile communications. The fact that an operator no longer needs to undertake any frequency planning means that there will be greater flexibility in deployment of the network. For example, the integration of micro-cellular layers in a macro-cellular network will be largely facilitated because there will no longer be any need to share the radio spectrum between the different layers of cells and between cells within a same layer.
The method allows the channels to be automatically planned between the cells. It ensures that the system is converged rapidly and automatically to a stable configuration in which the radio resources are correctly distributed between the cells (no channel overlap between two adjacent cells). Furthermore, this, DCA method is very reactive in terms of the various modifications which might be made in the network (changes in topology, traffic variations) since it knows how to modify the distribution of channels between cells accordingly. The DCA method proposed by the invention can therefore be used with all types of configuration used in a GSM or similar network.
The invention has the advantage of requiring no modification to the current signaling protocols of a network of the GSM type. In order to apply the invention, it is sufficient to implement the DCA method at the level of the BSC (base station controllers).
Although one of the advantages of the DCA mechanism is that no frequency planning (FCA) is needed, the DCA mechanism proposed by the invention is nevertheless not incompatible with frequency planning. A GSM operator can apply this DCA mechanism in a network which also uses a frequency plan. In fact, for each cell, the extent of the radio spectrum in which the DCA algorithm will use resources can be specified. This may be done simply by suitably selecting the set of physical channels subjected to the method. The invention will then allow FCA/DCA combinations of all types, bringing great flexibility in the deployment of the network.
A preferred implementation of the method according to the invention further comprises, for each base station, the step of comparing the priority indices of channels from the first and second lists in order to trigger a handover from a logical channel belonging to a busy physical channel from the second list to an accessible logical channel belonging to a physical channel from the first list whose priority index is higher than that of said busy physical channel from the second list
This improves the convergence speed of the DCA algorithm to a stable configuration. However, the main advantage is the possibility to optimise the communication quality on the channels used: if a cell tries to use a channel that is already being used by an adjacent cell, it will rapidly detect the presence of interference and choose another channel by means of the handover procedure (automatic intra-cellular transfer of communication).